Stirrer for use in liquid storage tanks, and a method for determining its position in the tank

ABSTRACT

A device with vertical buoyancy tanks fitted to its upper part is provided with rotors which generate horizontal and vertical propulsion, and which stirs the liquid uniformly as it moves freely through a liquid storage tank, thereby preventing the accumulation of sludge. The vertical buoyancy tanks stabilize the stirrer and provide a restoring force if the stirrer is upset. The stirrer is fitted with an ultrasonic wave emitter, and ultrasonic receptors are installed at several positions in the liquid tank. An ultrasonic pulse is emitted after a certain interval when the stirrer is at rest on the bottom of the tank. The position of the stirrer is computed for each pulse emission, and the average is taken to give the actual position of the stirrer.

This is a division of application Ser. No. 861,012, filed May 8, 1986.

BACKGROUND OF THE INVENTION

This invention relates to a device for stirring liquid in a storage tankwhile moving around in the tank, thus redissolving sludge which hasseparated out from the liquid, and preventing the accumulation of sludgeon the bottom of the tank. It also relates to a method for determiningthe position of the stirrer in the tank so that the motion of thestirrer may be controlled.

In large tanks used for storage of liquid, (for instance, crude oil,residual oil, etc.) sludge may gradually separate out from the liquidover long periods of time. If this sludge is allowed to build up on thebottom of the tank, the quantity of liquid which can be stored in thetank is effectively reduced, and the tank must therefore be cleanedregularly to remove accumulated sludge. In order to clean the tank, itis necessary to drain off all the liquid. A device was thereforedeveloped to obviate this necessity, consisting of a stirrer rotorinside the tank rotated by a motor outside the tank, which stirred theliquid and prevented sludge from separating out. The position of thesestirrer rotors was however fixed, which meant that similar device had tobe installed at a large number of points around the circumference of thetank. This solution is not practical and does not permit efficientstirring in the center area of the tank.

In order to solve this problem, the Applicant, in Japanese PatentPublication No. 59-46658 and Japanese Patent Provisional Publication No.58-214383, proposed a device which stirred the liquid while movingaround in the tank. This stirrer has rotors which permit it to move upand down, and rotors which permit it to move in a horizontal direction.By controlling these rotors from outside, the stirrer can be made tomove around freely inside the tank. The liquid in the tank can thereforebe stirred uniformly, sludge can be redissolved, and sludge can beprevented from accumulating anywhere on the bottom of the tank.

In order to lighten the weight of the stirrer and enable it to movesmoothly through the liquid, it was suspended by a rope from a float togive it buoyancy. However, as the stirrer was suspended from the float,the stirrer sometimes lost its balance when it was moved horizontally orrotated, and sometimes collided with the tank inner wall.

Further, in order to move the stirrer to a desired location somewhere ina large tank by means of an external operation, it is necessary todetermine the actual position of the stirrer at any time. As liquidstorage tanks are usually completely enclosed, however, the position ofthe stirrer cannot be confirmed from outside.

The Applicant, in Japanese Patent Provisional Publication No. 58-213210,proposed a device for determining the position of the stirrer wherein awire wound around a drum was suspended from the roof of the storage tankand connected to the upper part of the stirrer, the direction and lengthof wire released, as measured by a potentiometer or other means, beingused to determine the position of the stirrer.

The structure of this device is however complex, and if the directionand length of wire released are not measured with a high degree ofprecision, it is difficult to determine the position of the stirrer withaccuracy. Further, as the wire is paid out over a long distance, itsometimes goes slack or becomes entangled with the electric cable of thestirrer.

SUMMARY OF THE INVENTION

The object of the present invention, therefore, is to provide a stirrerwhich can move smoothly in both horizontal and vertical directions inthe liquid in the tank, and which is stable to change of direction.

A further object of this invention is to provide a method fordetermining the precise position of the stirrer.

For these purposes, vertical buoyancy tanks are fitted to the upper partof the frame which constitutes the skeleton of the stirrer, and aperpendicular flow plate is provided on the lower part of the frame. Apair of motor-driven horizontal rotors are arranged on either side ofthe flow plate in order to provide propulsion in a horizontal direction.In addition, an elliptical housing for enclosing a pair of verticalrotors, also motor-driven, is fitted to the lower part of the flow platewhich provide propulsion in a vertical direction. The rotation of themotors is controlled by a control unit connected to the stirrer by acable. The flow set up in a horizontal direction by the horizontalrotors causes the stirrer to move horizontally in the opposite directionto the flow, while the flow set up in a vertical direction by thevertical rotors causes the stirrer to rise and fall in the oppositedirection to the flow. At the same time, a flow of liquid in set up inthe tank, and this stirring action causes sludge to redissolve in theliquid. Due to the vertical buoyancy tanks fitted to the upper part ofthe frame in a one-piece construction, there is a strong vibrationdamping action and a restoring action which prevent the stirrer frombeing upset and maintain it in its correct orientation.

Due to the floats, the weight of the stirrer in the liquid ispractically zero. The driving force required to lift the stirrer is thusreduced and, by decreasing the speed of the stirrer when it is falling,a collision with the bottom of the tank is prevented.

The flow plate is parallel to both the horizontal and verticaldirections of the rotors, which stabilizes the orientation of thestirrer when it is moving.

In this invention, in order to determine the position of the stirrer inthe liquid storage tank, an ultrasonic wave emitter is fitted to thestirrer, and several receptors are installed at different points insidethe tank to receive the ultrasonic wave so emitted. A computing deviceis provided to compute the position of the stirrer, which functions bydetermining the time taken by each receptor to receive this ultrasonicpulse. The time taken to receive the pulse from the ultrasonic waveemitter is directly proportional to the distance to each receptor, andso the position of the stirrer can be found from these times. To reduceerrors due to noise when the measurements are being carried out, allmotors are stopped, and the stirrer allowed to settle on the tank bottombefore emitting an ultrasonic pulse from the emitter. This emission isrepeated several times. The period between emission of pulses is chosento be greater than a time interval equivalent to the time taken totraverse the greatest distance inside the tank plus the time taken forattenuation of echos due to reflections from the tank inner wall. Asmeasuring errors arise if the echos of the ultrasonic pulses are used,distances are measured using the time taken for the first pulse to bereceived in each emission cycle. The position of the stirrer is computedfor pulses in several cycles, and the average value obtained is taken tobe the actual position.

The above object, other objects and advantages of this invention will bemade clearer by the following description and reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of the stirrer of this invention.

FIG. 2 is a side view of the stirrer of this invention.

FIG. 3 is a plan view of the stirrer taken on the line A--A in FIG. 2.

FIG. 4 is a vertical sectional view through a liquid storage tankcontaining the stirrer of this invention.

FIG. 5 is a diagrammatic view of the parts involved in determining theposition of the stirrer.

FIG. 6 is a waveform diagram showing the relation between the ultrasonicpulse emitted and the pulse received.

FIG. 7 is a waveform diagram showing the relation between the ultrasonicpulse and the echos inside the tank.

FIG. 8 is a vertical sectional view through a tank showing how thestirrer cable is connected.

FIG. 9 is a waveform diagram showing control of the stirrer rotors, andcontrol of ultrasonic pulse emission.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As can be seen from FIGS. 1-3, this device consists of a vertical frame1, and an elliptical housing 2 fixed to the lower part of the frame, thehousing 2 being provided with several windows 2a. These elementstogether form frame 4 which is the skeletal structure of the stirrer 3.

On the upper part of frame 1, a motor 5 is provided which drives thevertical rotors 6a, 6b arranged symmetrically on either side ofelliptical housing 2. Rotation is transmitted to rotors 6a, 6b by meansof gearboxes 5a, 5b connected to the shaft of motor 5. Rotors 6a, 6bconsist of impellers with several blades, and their rotation sets up apractically perpendicular fluid flow. On either side of the motor 5,there are a pair of motors 7a, 7b, and gearboxes 8a, 8b installed invertical frame 1. On either side of frame 1, there are also horizontalrotors 9a, 9b oriented so as to set up a practically horizontal fluidflow. These rotors 9a, 9b are driven by motors 7a, 7b.

Rotating brushes 11a, 11b are arranged via bearing boxes 10a, 10b in thelower part of elliptical housing 2, and are connected to the shafts ofrotors 6a, 6b respectively such that they rotate together with them.

On the upper part of frame 4, a pair of primary buoyancy tanks 12a, 12b,which have a semicylindrical form and are hollow, are arranged so as toenclose between them the upper part of frame 1 and the motors 5, 7a, 7b.The lower parts of the tanks are fixed to frame 1, and the upper partsare fixed in a one piece construction to the joint box 13 provided atthe upper end.

In addition a secondary buoyancy tank 14, which has an ellipticcylindrical form, is vertical and is of large volume, is positioned onthe upper part of joint box 13 to which it is fixed as one piece

The upper surfaces of primary buoyancy tanks, 12a, 12b slant away fromframe 1 towards the exterior so as not to offer any resistance to thevertical flow of fluid set up by said rotors 6a, 6b.

Between primary buoyancy tanks 12a, 12b, frame 1 and elliptical housing2, a flow plate 15 is arranged at right angles to frame 1, that is,parallel to the fluid flow set up by both the rotors 6a, 6b and therotors 9a, 9b. This flow plate 15 is fixed to tanks 12a, 12b, frame 1and elliptical housing 2, and also serves to provide greater strength.

Several obstacle detectors 16 are installed on the outer wall ofelliptical housing 2. These obstacle detectors 16 use tape switches,limit switches or pressure sensitive switches, or any other commondevice which emits a signal when it comes into contact with an obstacle.

A depth finder 17 is fitted to joint box 13, and may for example consistof a strain gauge, potentiometric gauge or Bourden tube pressure gauge.It is so designed as to output a depth signal due to the pressuredetected when the stirrer 3 is immersed in the liquid in the tank.

A position finding device is provided, as shown in FIGS. 4 and 5, fordetermining the position of the stirrer in the liquid in the tank.

This position finding device involves the use of an ultrasonic wavesystem. An ultrasonic emitter 19, for example of the cylindricalpiezo-electromagnetic type, is fitted to the center of the upper part ofsecondary buoyancy tank 14. Three receptors 20a, 20b, 20c, which areintended to receive the ultrasonic wave pulse and are suspended from theroof 27 of tank 22, are then placed in their respective positions.

As can be seen from FIGS. 5 and 6, when an ultrasonic pulse is emittedat fixed time intervals from the emitter 19, the time (t) required forthis pulse to reach the receptors 20a, 20b and 20c (t₁, t₂, t₃), isdirectly proportional to the distance from the emitter to the receptor.The distance from emitter 19 to each of the receptors 20a, 20b and 20c,that is l₁, l₂, l₃, may then be measured by multiplying the respectivetimes t₁, t₂, t₃ recorded by measuring instrument 32, by the velocity ofsound. A computer 33 is provided to compute the position of stirrer 3 inthe liquid storage tank from the distances l₁, l₂, l₃.

As shown in FIG. 5, if the position of the ultrasonic emitter 19 inthree-dimensional coordinates is (x, y, z), the positions of theultrasonic receptors 20a, 20b, 20c in the same coordinate system are(x₁, y₁, z₁), (x₂, y₂, z₂), (x₃, y₃, z₃), and the distances l₁, l₂, l₃are determined, the point (x, y, z) may be found from the followingequations (1)-(3):

    (x-x.sub.1).sup.2 +(y-y.sub.1).sup.2 +(z-z.sub.1).sup.2 =l.sub.1.sup.2(1)

    (x-x.sub.2).sup.2 +(y-y.sub.2).sup.2 +(z-z.sub.2).sup.2 =l.sub.2.sup.2(2)

    (x-x.sub.3).sup.2 +(y-y.sub.3).sup.2 +(z-z.sub.3).sup.2 =l.sub.3.sup.2(3)

If the receptors 20a, 20b, 20c are arranged at the same height (z₁ =z₂=z₃), the computation is simplified.

The ultrasonic emitter 19 is made to emit several pulses separated by agiven time internval, and the computer 33 performs the computationdescribed above for each pulse.

By repeating this computation several times and taking the average, itis possible to obtain an accurate measurement for the position of thestirrer with very little error. If, however, motors 5, 7a and 7b arerunning when the ultrasonic pulse is emitted, errors of measurement caneasily arise due to noise. All motors are therefore first stopped, andthe pulse is emitted after a certain time has elapsed, that is, afterthe stirrer has settled on the bottom of tank 22. In carrying out themeasurement, the interval of ultrasonic emission is determined by themaximum value of the distance to be measured (equivalent to the diameterof the tank). If however there are considerable echos due to collisionof the ultrasonic wave with and reflection from the tank inner wall, theinterval is determined as shown in FIG. 7 by time consisting of the times₁ corresponding to the greatest distance of traverse, plus the time s₂required for the echos to absorb completely.

Even in this case, however, the ultrasonic receptors 20a, 20b, 20c mayreceive echos after they have received the first pulse until the nextpulse emission begins. The measuring instrument 32 therefore treats thefirst pulse received as correct information, and ignores the subsequentechos. This is because the shortest distance between the emitter 19 andthe receptors 20a, 20b, 20c is the straight line distance connectingthem, and echos which do not take this path but are reflected mustnecessarily be received after the regular pulse has arrived.

The motor drive and signal cable 21 attached to the stirrer 3 is wiredalong the secondary buoyancy tank 14. This cable 21, as shown in FIG. 4,is supported by floats 24, 25 positioned in the liquid 23 contained intank 22, and is connected to the control unit 26 outside tank 22.

If the roof 27 of the liquid tank 22 is of the floating type, the endsof the floats 25, as shown in FIG. 8, come into contact with roof 27.The length l of the floats 25 is greater than the length l₁ of thecolumns 28, so that cable 21 does not become tangled around columns 28which support floating roof 27 when it reaches the bottom of the tank.If a cable duct 29 is provided for cable 21 to the floating roof 27, thelength l of the floats 25 is of course chosen to be greater than thelength l₂ of the duct.

The control unit 26 determines the position of stirrer 3 when the aboveposition finding device is operated. It also drives motors 5, 7a, 7b andcontrols the motion of the stirrer based on the position found, thecontact signals received from said obstacle detectors 16, and the depthsignal from depth finder 17.

Alarm indicators are provided on the control unit to show the status ofstirrer position, depth and contact with obstacles. A manual operationswitch and auto operation switch are provided so that the controloperations can be performed either manually or automatically as desired.

As shown in FIG. 4, the stirrer is introduced into tank 22 through amanhole 30 in the roof. When the stirrer is introduced, it is adjustedby means of the primary and secondary buoyancy tanks 12a, 12b and 14,such that is weight in the liquid is practically zero.

The motors 5, 7a, 7b are started from control unit 26 and, by rotatingthe vertical rotors 6a, 6b and the horizontal rotors 9a, 9b in a givendirection or in the reverse direction, the stirrer is made to rise,fall, move forwards or backwards horizontally, or change its direction.

If the vertical rotors 6a, 6b are rotated clockwise, liquid is pushedvertically downwards by the impeller blades, and stirrer 3 thereforerises. If rotors 6a, 6b are rotated in a reverse direction, liquid ispushed vertically upwards and stirrer 3 falls.

If the horizontal rotors 9a, 9b are rotated clockwise, liquid 23 ispushed back horizontally, and the stirrer moves forward. If rotors 9a,9b are rotated counter-clockwise, liquid 23 is pushed forwards, andstirrer 3 moves backward. Further, if rotors 9a, 9b are rotated inmutually opposite directions, the direction of the stirrer changes.

Thus, as the stirrer moves almost uniformly through liquid storage tank22, the rotors 6a, 6b or rotors 9a, 9b stir the liquid at the same time.This causes the sludge which tends to separate out from the liquid andaccumulate on the bottom of storage tank 22 to be stirred, lifted up bythe flow, dispersed, and prevented from accumulating on the bottom.Further, when the stirrer is on the bottom of tank 22, the rotatingbrushes 11a, 11b are driven to mechanically pulverize the sludge andshear through it. The sludge thus becomes finer, floats up and iscarried on the upward flow produced by rotors 6a, 6b, tending to promoteits redissolution in the liquid.

As has already been mentioned, vertical buoyancy tanks 12a, 12b and 14are fitted to frame 4 in a one piece construction. As a result, when thestirrer 3 is moving forwards or backwards and loses its balance oroscillates, there is a strong vibration damping effect and restoringforce tending to maintain its orientation stable.

Due to buoyancy tanks 12a, 12b and 14, the weight of stirrer 3 in theliquid is practically zero. It does not therefore require such largedriving force to make the stirrer rise. Similarly, there are no suddenfalls when the stirrer is falling, and damage due to collision of thestirrer with the bottom of storage tank 22 is avoided.

The stirrer 3 is also provided with a flow plate 15. When the stirrer ismoving forwards or backwards, therefore, its direction is constrained bythis flow plate 15, so that motion is executed in a perfectly straightline.

This sequence of operations can be performed manually while watchingvarious indicators on control unit 26, and automatically by means of acontrol program set up in control unit 26.

In the case of automatic operation, control unit 26 first lifts thestirrer from a resting position on the bottom of the tank, moves ithorizontally by a given distance, allows it to settle on the bottomagain, and stirs the liquid. This normal control sequence is effected asfollows.

Motors 5, 7a and 7b are driven so as to rotate vertical rotors 6a, 6bclockwise, and horizontal rotors 9a, 9b both clockwise. These rotationscause the stirrer to move forwards while rising. After rotors 6a, 6bhave been rotated clockwise for a time T₁, they are rotated in a reversedirection for a time T₂ (T₁ <T₂), and rotors 9a, 9b are stopped after atime T₃.

These operations cause the stirrer 3 to move forward a given distanceand to fall to the bottom of the tank 22. The rotating brushes 11a, 11bare then rotated so as to remove sludge 31 adhering to the bottom of thetank and clean the bottom. After the rotors 6a, 6b have rotated in thereverse direction for time T₂, control unit 26 again rotates rotors 6a,6b clockwise for a time T₁, and rotates rotors 9a, 9b for a time T₃. Byrepeating these operations over and over again, the stirrer 3 is made torise, fall and move forward stirring and cleaning liquid storage tank 22as it does so.

If, during the above operations, a signal is received from the obstacledetectors 16 indicating that the stirrer has come into contact with anobstacle or with the inner wall of tank 22, evasive action is taken.

This evasive action consists of lifting the stirrer from its positionand rotating it, the direction of this rotation and the magnitude of theangle of rotation being completely random. If this action were givensome regularity, the stirrer's motion would fall into a steady pattern(such as, for example, executing a back and forth motion betweenopposite walls), which it is intended to avoid. This control sequence iseffected as follows.

If rotors 6a, 6b are rotating counterclockwise, control unit 26 drivesmotor 5 so as to rotate them clockwise; while if rotors 6a, 6b arerotating clockwise, motor 5 is driven so as to continue this clockwiserotation. At the same time, motors 7a, 7b are driven so as to rotaterotors 9a, 9b in the opposite direction to one another. Due to theclockwise rotation of rotors 6a, 6b, and the rotation of rotors 9a, 9bin mutually opposite directions, stirrer 3 rises while rotating aboutits own axis.

The stirrer is made to rise for a time T₄, following which rotors 6a, 6bare rotated counterclockwise for a time T₅ (T₄ <T₅), and rotors 9a, 9bare stopped after a time T₆ (T₆ <2T₄). Due to these operations, thestirrer body rotates, and then falls and settles on the bottom of thetank. The direction of this rotation is determined by the mutuallyopposed directions of rotation of rotors 9a, 9b, and the angle ofrotation is determined by T₆. The choice of direction and the magnitudeof T₆ depend on the signals from the random signal generatorincorporated in control unit 26.

If control unit 26 continues to receive signals from the obstacledetectors 16 even after the end of period T₆, the above controloperations are repeated until no further signals are received. When nofurther signals are received, the stirrer is operated normally asdescribed previously.

Control unit 26, apart from carrying out normal control and obstacleevasion control, also determines the depth and position of the stirrerby receiving signals from depth finder 17 and the position findingdevice. The range of movement of the stirrer can be set depending on itsdepth and position.

The position of the stirrer is determined as described previously. Incarrying out the determination, control unit 26 operates the positionfinding device when the stirrer is at rest, as shown by FIG. 9. As shownin FIG. 9, the stirrer body 3 rises when rotors 6a, 6b are rotatingclockwise. When rotation is stopped, however, the stirrer body 3 fallsunder its own weight, and settles on the bottom of the tank after a timeT₀. After a further time T'₀, an ultrasonic wave pulse is emitted fromultrasonic emitter 19 in the position finding device, and the positionof the stirrer is determined. This determination is carried out with thestirrer stationary on the bottom of the tank in order to eliminate theeffect of noise due to motors 5, 7a and 7b, and to give a stable readingfor the position.

When the stirrer reaches the limit of the set range of movement,depending on the position found, control unit 26 rotates rotors 6a, 6band rotors 9a, 9b, and changes the direction of stirrer 3 as in theobstacle evasion sequence. If stirrer 3 was rising, it is made to fall.By setting the range of movement beforehand, therefore, the stirrermoves only over the set range, and stirs the liquid in the tankefficiently.

What is claimed is:
 1. A method for determining the position of amotor-driven stirrer which is movable through a liquid in a tank whilestirring such liquid, comprising the steps of placing an ultrasonic waveemitter on said stirrer, arranging a plurality of receptors at differentlocations in said tank, operating motors on the stirrer to move saidstirrer within said tank, stopping said motors such that said stirrerrests on the bottom of said tank in a first position, emittingultrasonic pulses from said emitter to said receptor while said stirrerrests on the bottom of said tank in said first position, measuring thetime required for said pulses to reach each of said receptors, the timerequired for said pulses to reach each receptor being related to thedistance between said emitter and each of said receptors, disposing acomputer outside of said tank, and calculating said first position ofsaid stirrer in said tank based on said measured times by feeding thelatter to said computer which computes the position of said stirrer insaid tank.
 2. A method according to claim 1 further comprising emittinga plurality of pulses from said emitter separated by a time interval. 3.A method according to claim 2 further comprising measuring the pluralityof times required for each of said plurality of pulses to reach each ofsaid receptors, performing a plurality of position calculations based onsaid plurality of times, and utilizing the average of said plurality ofposition calculations to accurately determine the position of saidstirrer in said tank.
 4. A method according to claim 2 wherein said timeinterval is greater than the time required for the ultrasonic pulses totraverse the maximum dimension of said tank.
 5. A method according toclaim 2 wherein said time interval is greater than the time required forthe ultrasonic pulse to traverse the maximum dimension of said tank plusthe time required to absorb echoes reflected from the inner wall of thetank.
 6. A method according to claim 1 further comprising utilizing onlythe first pulse detected in each pulse cycle for calculating saidposition.
 7. A method according to claim 1 further comprising operatingsaid motors to move said stirrer to a different location within saidtank, stopping said motors such that said stirrer rests on the bottom ofsaid tank in a second position, and repeating the aforesaid steps tocompute the second position of said stirrer in said tank.
 8. A methodaccording to claim 1 further comprising detecting the depth of saidstirrer in said liquid in said tank.
 9. A method according to claim 1further comprising detecting when said stirrer contacts an obstacle insaid tank.
 10. A method for determining the position of a stirrer whichis movable through a liquid in a tank, comprising the steps of placingan ultrasonic wave emitter on said stirrer, arranging a plurality ofreceptors at different locations in said tank, emitting ultransonicpulses from said emitter to said receptors while said stirrer is in afirst position in said tank, measuring the time required for said pulsesto reach each of said receptors, the time required for said pulses toreach each receptor being relates to the distance between said emitterand each of said receptors, disposing a computer outside of said tank,and calculating said first position of said stirrer in said tank basedon said measured times by feeding the latter to said computer whichcomputes the position of said stirrer in said tank.
 11. A methodaccording to claim 10 wherein said step of calculating said firstposition of said stirrer comprising calculating the three dimensionalcoordinates representing said first position.